Well believe me.... you could very well be on to SOMETHING. Trust your intuition to investigate, but you need to confirm it. I'll get to the point on this.... if you want to discover if your partner is fibbing on you or not, just check the body language. Few trained professionals can fake body language well (trust me, it takes hypnosis to really get lying down as a body language art).
Most people lie by focusing on their vocal tone and straightforwardness of answer. This takes away their focus from the body language aspect. They can't focus on all these things right at the moment. This will give away at least one or two signs for you catch them hard.
The first thing to do is to ask them a "trick" question about their activity or whereabouts. The response is to get them to answer "correctly" and see if this answer is hiding a lie.
Here are the signs to be keen of....
1. Fingers or hand over the mouth
2. Fingers of hand over an ear
3. Fingers or hand over the eyes
Any of these can give away a strong signal if this is done right at the moment of a lie.
Now, remember, the story does get deeper because there are 2 micro-facial expressions that can confirm this. See the highly-acclaimed full-proof Cheat Sweeper system for these...